Microactuator having a ferromagnetic substrate

ABSTRACT

An electromagnetic microactuator having a ferromagnetic substrate, preferably steel. A plurality of layers are deposited upon the substrate including an insulating layer, a seed layer and first and second photoresist layers. The first photoresist layer provides a coil well which defines the deposition location of a coil metal. The second photoresist layer provides central and peripheral core wells which define the deposition locations of a central core and a peripheral core, respectively. The second photoresist layer intersticially fills and protectively covers the coil.

TECHNICAL FIELD

The present invention relates generally to microactuators and morespecifically to an electromagnetic microactuator having a ferromagneticsubstrate and a method for making same.

BACKGROUND OF THE INVENTION

There are a variety of microactuators in the art based on electrostatic,thermal mechanical, piezoelectric, shape memory alloy or electromagneticactuation principles. For automotive applications, microactuators arerequired to have large displacement (in the tens of micrometers), wideoperational temperatures (from minus 40 to plus 125 degrees Celsius) andlow operational voltages (12V). Under such requirements, electromagneticactuation is the best choice. An electromagnetic microactuator includesan inductive component that generates a magnetic flux and a magneticcore to guide the magnetic flux. Construction of electromagneticmicroactuators include the use of AZ400 series positive photoresists orphotosensitive polyimide to form the plating mold. However, the AZ400phtotoresist has an aspect ratio of less than 3 and poor planarization.

Silicon wafers are used as a substrate for electromagneticmicroactuators. Using a silicon wafer as the substrate requires a longprocessing time. Five hours is required for a 300 micrometer deepcavity, and electroplating of the bottom return core takes 10 hours for300 micrometer thick permalloy. There is also a large thermal expansioncoefficient mismatch among the copper, permalloy and silicon components.The thermal expansion coefficients of copper, permalloy and silicon are17, 15 and 3 ppm/degree Celsius, respectively. The differences inthermal expansion may cause difficulties in device fabrication and havea detrimental effect on device performance.

By way of instructive example, FIG. 1 shows a prior art silicon-typeelectromagnetic microactuator 100 fabricated with a silicon wafersubstrate 102. The electromagnetic microactuator 100 further includes acenter core 104, a peripheral core 106, a spiraling copper coil 108 andterminal pads 110 connected to the coil ends. The silicon substrate 102has a cavity 112 formed therein whereat is located a flux return pathcore 114 fabricated from nickel-iron. A silicon dioxide layer 116 isformed on the silicon substrate 102 before application of the centercore 104, the peripheral core 106, the copper coil 108 and the terminalpads 110. The center core 104, the peripheral core 106, the copper coil108 and the terminal pads 110 are formed on the silicon dioxide layer116 with microelectroforming techniques. The center and peripheral coresare fabricated from nickel/iron. An SU-8 masking material 118 is alsoused. A ferromagnetic plate-shaped armature 120 is disposed adjacent theelectromagnetic microactuator 100. When electrical current is runthrough the copper coil 108, magnetic flux is generated in the centerand peripheral cores in cooperation with the return flux path core 114,resulting in an attractive magnetic force F_(M) applied to the armature120. This magnetic force causes the armature to move toward theelectromagnetic microactuator 100, overcoming opposed biasing (as forexample by a return spring).

SUMMARY OF THE INVENTION

The present invention is an electromagnetic microactuator having aferromagnetic substrate and a method for making same, which providesbetter planarization and aspect ratios than that of the prior art, andfurther simplifies fabrication.

The method of fabrication of an electromagnetic microactuator accordingto the present invention includes a ferromagnetic substrate (base),preferably steel, and a plurality of mask deposited layers.

A first processing step includes a spin coat of spin-on-glass upon thesubstrate, patterned using a contact mask. A second processing stepincludes depositing a seed layer of titanium-tungsten-gold, patternedusing an anchor mask. A third processing step includes depositing asacrificial layer of chromium-aluminum, patterned using a plating mask.A fourth processing step includes a spin coat of an SU-8 photoresistfirst mold layer, patterned using a copper coil mask. A fifth processingstep includes electroplating with copper. A sixth processing stepincludes stripping the SU-8 photoresist first mold layer with plasmaetching. A seventh processing step includes stripping the sacrificiallayer by an etching solution, an eighth processing step includes a spincoat of an SU-8 photoresist second mold layer, patterned using a coremask, which intersticially fills and covers the coil. A ninth and finalprocessing step includes electroplating center and peripheralnickel-iron cores.

Accordingly, it is an object of the present invention to provide amethod for fabrication of an electromagnetic microactuator, whichsimplifies fabrication by utilization of a ferromagnetic substrate.

This and additional objects, features and advantages of the presentinvention will become clearer from the following specification of apreferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art electromagneticmicroactuator fabricated on a silicon wafer adjacent a ferromagneticplate-shaped armature.

FIG. 2 is a cross-sectional view of a microactuator according to thepresent invention at a first step of fabrication, showing aferromagnetic substrate with a layer of spin-on glass.

FIG. 3 is a cross-sectional view of the microactuator according to thepresent invention at a second step of fabrication involving a seed layerof deposited titanium-tungsten-gold.

FIG. 4 is a cross-sectional view of the microactuator according to thepresent invention at a third step of fabrication involving a depositedsacrificial layer of chromium-aluminum.

FIG. 5 is a cross-sectional view of the microactuator according to thepresent invention at a fourth step of fabrication involving a spincoated SU-8 photoresist first mold.

FIG. 6 is a cross-sectional view of the microactuator according to thepresent invention at a fifth step of fabrication involving copperelectroplated into the SU-8 photoresist first mold.

FIG. 7 is a cross-sectional view of the microactuator according to thepresent invention after a sixth step of fabrication involving removal ofthe SU-8 photoresist first mold and at a seventh step of fabricationinvolving removal of the sacrificial layer.

FIG. 8 is a cross-sectional view of microactuator according to thepresent invention at an eighth step of fabrication involvingspin-coating and patterning a SU-8 photoresist second mold.

FIG. 9 is a cross-sectional view of the microactuator according to thepresent invention at a ninth step of fabrication involving deposition ofnickel-iron central and peripheral cores.

FIG. 10 is a top view of a ferromagnetic substrate electromagneticmicroactuator according to the present invention fabricated according tothe steps of FIGS. 2 through 9.

FIG. 11 is a cross-sectional view of the electromagnetic microactuator,seen along line 11-11 of FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the Drawings, FIGS. 10 and 11 depict an example of anelectromagnetic microactuator 10 according to the present invention, andFIGS. 2 through 9 depict fabrication steps for the microactuator.

The method for fabrication of the electromagnetic microactuator 10includes providing a ferromagnetic substrate 12 and depositing aplurality of masked layers thereupon. The ferromagnetic substrate servesas a return flux path core and is preferably fabricated from steel, butother materials having ferromagnetic properties may also be used.

FIG. 2 shows a first processing step for forming the electromagneticmicroactuator 10. A spin-on-glass insulating layer 14 is spin-coated andpatterned onto the ferromagnetic substrate 12 using a contact mask (notshown). The insulating layer 14 preferably has a thickness of 1.8micrometers. The contact mask leaves a central well 16 and a peripheralwell 18. Preferably, the spin coating process of the insulating layer 14includes the following attributes. The spin coating is preferablyapplied at a spin speed of 2000 rpm for 30 seconds. The spin-on-glass isheat treated with a hot plate in air at 90 degrees Celsius for a periodof 2 minutes; at 150 degrees Celsius for a period of 2 minutes; and at250 degrees Celsius for a period of 2 minutes. The spin-on-glass ispreferably patterned photolithographically and etched with a bufferedhydrofluoric acid. However, other methods of applying the spin-on-glassto the substrate 12 may also be used and other insulating layers otherthan spin-on-glass may also be used.

FIG. 3 shows a second processing step for forming the electromagneticmicroactuator 10. A seed layer 20 of titanium-tungsten-gold is depositedand patterned using an anchor mask (not shown). The seed layer 20 ispreferably deposited by a sputtering method and patternedphotolithographically. The gold is preferably etched with a commerciallyavailable wet chemical solution and the titanium-tungsten is preferablyetched with 50 degree Celsius hydrogen peroxide. However, other methodsof applying the seed layer 20 may also be used and other seed layershaving a different compositions may also be used.

FIG. 4 shows a third processing step for forming the electromagneticmicroactuator 10. A sacrificial layer 24 of chromium-aluminum 24 isdeposited and patterned using a plating mask (not shown). Thesacrificial layer 24 is preferably deposited using E-beam evaporation,patterned photolithographically and etched by a commercially availablewet chemical solution. However, other compositions and depositionmethods may be used for the sacrificial layer.

FIG. 5 shows a fourth processing step for forming the electromagneticmicroactuator 10. An SU-8 photoresist first mold layer 26 is applied tothe sacrificial layer 24 and patterned using a copper coil mask (notshown). The SU-8 photoresist first mold layer 26 serves to define aspiraling coil well 28 and two terminal pad wells 30 (only one terminalwell being shown for purposes of clarity). The SU-8 photoresist firstmold layer 26 is preferably applied by spin coat at a spin speed of 950rpm for 20 seconds to obtain an approximate 60 micrometer thickness. TheSU-8 photoresist first mold layer 26 is preferably pre-baked in air on ahot plate at 93 degrees Celsius for 18 minutes, preferably exposed toultraviolet light at an intensity of 10 mW/cm² for 120 seconds,preferably post baked on a hot plate at 95 degrees Celsius for 8minutes, and, finally, preferably developed with a spin-spray system for4 minutes and nitrogen dried for 1 minute.

FIG. 6 shows a fifth processing step for forming the electromagneticmicroactuator 10. A conductive coil 32 and first and second terminalpads 34 a, 34 b (as shown in FIG. 10) are preferably formed byelectroplating copper into the coil well 28 and the terminal pad wells30, respectively. The first and second terminal pads 34 a, 34 b have agreater height than the conductive coil 32 due to the feature sizedependent of electroplating rate. The greater height of the two terminalpads 34 is a benefit for soldering thereto external circuit leads. Thecopper electroplating is preferably implemented using the followingsubsteps and parameters. The plating solution is a copper sulfate basedacid having a temperature of 25 degrees Celsius. The plating rate is 20micrometers/hour. The plating current is approximately 0.4 amperes. Theplating solution is agitated by a magnetic bar stirrer. However, otherelectrically conductive metals may be used other than copper.

The first terminal pad 34 a is electrically connected to an outer end 32a of the coil 32. The second terminal pad 34 b is electrically connectedto an inner end 32 b of the coil 32 via the conductivity steel substrate12. The connection of the inner end 32 b to the second terminal pad 34 bcan be understood by reference to FIG. 11, which shows that the secondprocess step included depositing the seed layer 20 at locally raisedconnection locations 32 c, 34 c and serves as an electrically conductiveconnection between the inner end 32 b and the substrate 12 and betweenthe terminal pad 34 b and the substrate.

FIG. 7 shows sixth and seventh processing steps for forming theelectromagnetic microactuator 10. In the sixth step, the SU-8photoresist first mold layer 26 is stripped off, preferably using plasmaetching. In the seventh step, the sacrificial layer 24 is stripped off,preferably using selective etching. The etching is preferablyimplemented using the following substeps and parameters. Sixth stepplasma etching is implemented with 250 watts of power, an etch rate of 1micrometer/min and with an O₂/C₂F₆ etching gas at a flow rate of 100/25sccm. The selective etching of the seventh step is implemented with asolution of potassium ferricyanide.

FIG. 8 shows an eighth processing step for forming the electromagneticmicroactuator 10. An SU-8 photoresist second mold layer 36 isspin-coated and photolithographically patterned with a core mask (notshown) to form a central core well 38 and a peripheral core well 40.

FIG. 9 shows a ninth and final processing step for forming theelectromagnetic microactuator 10. The central core well 38 iselectroplated with nickel iron to form a center core 42 and theperipheral core well 40 is electroplated with nickel/iron to form aperipheral core 44. The electroplating of nickel/iron is preferablyimplemented using the following substeps and parameters. The platingbath is an IBM paddle cell, and the plating solution is a nickel sulfateand iron sulfate based solution having a pH of approximately 2.8. Theplating solution temperature is approximately 25 degrees Celsius; theplating current is 0.5 amperes; and the plating rate is 8micrometers/hour.

FIG. 10 shows a top plan view of the electromagnetic microactuator 10fabricated according to the above recounted process steps. FIG. 11 is across-sectional view showing in particular the electrical connection ofthe second terminal pad 34 b with the inner end 32 b of the coil 32 viathe substrate 12. FIG. 11 also shows how the SU-8 photoresist 36intersticially fills and protectively covers the coil 32 in a protectivemanner. As can further be seen from FIG. 11, when the coil 32 isenergized, the resulting magnetic field applies an attractive forceF_(M)′ upon an adjacent ferromagnetic plate armature 50, wherein returnflux passes through the central core 42, the peripheral core 44 and thesubstrate 12.

To those skilled in the art to which this invention appertains, theabove described preferred embodiment may be subject to change ormodification. Such change or modification can be carried out withoutdeparting from the scope of the invention, which is intended to belimited only by the scope of the appended claims.

1. An electromagnetic microactuator, comprising: a steel substrate; aninsulating layer selectively located on said substrate; a metallic seedlayer selectively located on said substrate and said insulating layer; acopper coil located on said seed layer, said coil having a spiral shapedefining a center and an outer periphery and being intersticially filledand covered by a photoresist; a central core of nickel iron located onsaid seed layer at said center of said coil; and a peripheral core offerrous material located on said seed layer adjacent said outerperiphery of said coil.
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